Method for producing joined body of ALN substrates and joining agent used for the joining

ABSTRACT

A novel method for joining aluminum nitride-series substrates to each other is provided in the substantial absence of an intervening third layer at the joining interface between the substrates. In the method, the aluminum nitride-series substrates are joined to each other by interposing a joining agent between the substrates heating the substrates and the joining agent to a first temperature range of at least the melting point of the joining agent to melt the joining agent and liquefy particles of the aluminum nitride at the neighborhood of the interfaces between the melted joining agent and the substrates, and then heating the joining agent and the substrates to a temperature range higher than the temperature range of the first process but lower than the melting point of the substrates to exhaust the joining agent from between the substrates.

This is a Division of application Ser. No. 08/941,388 filed Sep. 30,1997, now U.S. Pat. No. 6,028,022

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for producing a joined body ofAlN substrate and a joining agent used for the joining.

2. Related Art Statement

Heretofore, in conventional semiconductor production apparatuses, suchas, etching apparatuses, CVD apparatuses and the like, so-calledstainless heaters and indirect heating system heaters are generallyused. However, when these heating sources are used, there are problemsin that they are likely corroded by halogen-series corrosive gases toform particles thereon and that they have inferior heat efficiency. Inorder to solve such problems, the applicant previously disclosed in hisJapanese Patent Application Laid-Open No. 3-261,131 a ceramic heaterhaving a heating wire of high melting point embedded in a dense ceramicsubstrate, the wire being spirally wound in the interior of thesubstrate of a disc shape and connected at the both ends with anelectric terminal, respectively. Such a type of ceramic heaters arefound to have superior characteristic properties, particularly for theproduction of semiconductors.

As the ceramic materials constituting the substrate of the ceramicsheaters, silicon nitride, aluminum nitride, Sialon and the likenitride-series ceramic materials are considered preferable. In somecases, a susceptor is provided on the ceramic heater and a semiconductorwafer is mounted and heated on the susceptor. The applicant previouslydisclosed in his Japanese Patent Application Laid-Open No. 5-101,871that aluminum nitride is preferable as the substrate for such ceramicheaters and susceptors. This is because, particularly in semiconductorproduction apparatuses, ClF₃ and the like halogen-series corrosive gasesare often used as etching gases or cleaning gases, and aluminum nitridewas found to have an excellent corrosion resistant property to suchhalogen-series corrosive gases. Meanwhile, because ceramic materials aredifficult to process, researches have been made of joining ceramicmaterials of simple shapes to each other to obtain a ceramic element orpart of complicated shapes.

Nevertheless, at a joining interface between ceramic members a thirdphase having a different thermal expansion coefficient and mechanicalproperties is usually formed. The third layer has a problem in that itis usually easily broken by a thermal stress due to heating and coolingand various mechanical stresses. Particularly, in the case of aluminumnitride-series ceramics materials, the influence of the third layer wasserious, because of their low tenacity as compared with siliconnitride-series ceramic materials, etc.

If aluminum nitride-series ceramic materials are joined to each other bymeans of a glass or a compound consisting mainly of silicon, the thirdphase remaining on the joining interface is selectively corroded by aplasma of NF_(3,) ClF₃ or the like halogen series-corrosive gases. Thus,such joined bodies could not withstand the use under the corrosiveenvironment of the semiconductor production apparatuses.

There is also a method of directly joining the substrates made ofaluminum nitride sintered bodies to each other as described in JapanesePatent Application Laid-Open No. 2-124,778, wherein the substrates areheated at 1,800-1,900° C. and joined integrally by diffusion joining.However, in order to join the aluminum nitride sintered bodies by such adiffusion joining method, an extremely high temperature is necessary of,for example, 1,800-1,900° C. which is substantially the same hightemperature with the sintering temperature for producing the originalaluminum nitride sintered bodies. Henceforth, in the joining process thesubstrates are likely degraded and deformed. In addition, joined bodiesof a low strength of not more than about 60 MPa could only be obtained.

According to Japanese Patent Application Laid-Open No. 8-13,280, ajoined body of aluminum nitride sintered bodies is disclosed having arelatively high strength. However, in this method also, a substantiallythe same high temperature with the sintering temperature for producingthe aluminum nitride sintered bodies of the original substrates isrequired. In addition, super precise processing of the joining surfacesof the substrates of a roughness and a flatness respectively of not morethan 0.2 μm is required. Such a super precise processing increases theproduction cost.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a novel method ofproducing a joined body of aluminum nitride-series substrates to eachother in the substantial absence of an intervening third layer at thejoining interface of the substrates.

The present invention is a method of producing a joined body of aluminumnitride-series substrates, comprising a first step of interposing ajoining agent between the substrates to be joined, and heating thesubstrates and the joining agent at a temperature range of not less thanthe melting point of the joining agent to thereby melt the joining agentand liquefying particles of the aluminum nitride-series substrates to aliquid phase adjacent the interfaces between the melted joining agentand the substrates, and a second step of heating the substrates and thejoining agent at a temperature range of higher than the temperaturerange of the first step, but lower than the sintering temperature of thesubstrates to thereby exhaust the joining agent from between thesubstrates.

The inventors have made many researches on methods of joining thealuminum nitride-series ceramic materials at relatively low temperatureswithout interposing a third phase. For that purpose, the inventors havemade many experiments of interposing various metal oxides between thealuminum nitride-series ceramic materials and heating and melting themetal oxides. As a result, the inventors have found out that when thejoining agent is at first melted and then heated and held at atemperature higher than the melted temperature, the joining agent isexhausted from between the substrates thereby to form a firm joiningbetween the substrates. Observation of the interface of these substratesrevealed that the joining agent is substantially not remained to leave acontinuous texture of aluminum nitride substrates. In addition, theinventors have found out a finding leading to the present invention thatthe thus obtained joined body has very a high joining strength and muchsuperior airtight property and corrosion resistant property.

BRIEF EXPLANATION OF THE DRAWING

For a better understanding of the present invention, reference is madeto the accompanying drawings, in which:

FIG. 1a is a schematic front view of substrates with a joining agentinterposed therebetween showing the state before the joining thereof;

FIG. 1b is a schematic front view of the substrates 1 and 2 after thejoining thereof showing the state that the joining agent was melted inthe area of the interfaces of the substrates;

FIG. 2a is a schematic partial cross-sectional view of substrates with ajoining agent melted between the substrates;

FIG. 2b is a schematic partial cross-sectional view of the substratesshowing the state of the area of the joining interfaces between thesubstrates and the melted layer of the joining agent are melted;

FIG. 3 is a schematic partial cross-sectional view of an embodiment of astructure for holding a susceptor prepared by joining the susceptor anda circular retaining member;

FIG. 4 is a plan view of the holding structure of FIG. 3;

FIG. 5 is a graph of an example of pressure and temperature schedules;

FIG. 6 is a photograph taken by a reflective electron image showing aceramic tissue in the area of the interface of the joined body obtainedby Experiment 1 shown in Table 1;

FIG. 7 is a photograph taken by a reflective electron image showing aceramic tissue in the area of the joining interface of a bending testspecimen obtained by Experiment 2 shown in Table 1;

FIG. 8 is a photograph of a secondary electron image showing a ceramictissue at the neighborhood of the joining interface of the joined bodyafter treatment of the joined body of FIG. 7 by a heating cycle test;and;

FIG. 9 is a photograph of a secondary electron image showing a ceramictissue at the neighborhood of the joining interface of a bending testspecimen obtained by Experiment 1 shown in Table 1;

Numbering in the Drawings;

1, 2 substrate

1 a, 2 a face of the substrates 1, 2 to be joined

3 joining agent

4 expanded portion of melted layer of joining agent

5 melted layer of joining agent

6 susceptor

7 throughhole

8 holding member

9 inner space of the holding member 7

10, 11 electric cable

12 terminal of heat generating-resistive member

13 terminal of electrode

20 melted layer of joining agent

21 liquefied neighborhood of the interface between the substrates

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, the present invention will be explained in more detail withreference to attached drawings.

In the aluminum nitride-series ceramic material constituting thesubstrates, various sintering agents, coloring matters or otheradditives can be incorporated. As shown schematically in FIG. 1, a face1 a of a substrate 1 to be joined and a face 2 a of a substrate 2 to bejoined are opposingly arranged. At that time, a selected joining agent 3is interposed between the faces 1 a and 2 a of the substrates 1 and 2 tobe joined. Then, the substrates 1 and 2 and the joining agent 3 areheated to at least the melting point of the joining agent 3 to melt thejoining agent 3, as shown in FIG. 1b. A portion of the melted portion 20of the joining agent 3 is displaced, while wetting the surfaces 1 a and2 a of the substrates 1 and 2, to form an outer expanded portion 4, asshown in FIG. 1b.

In the early stage of the melting of the joining agent 3, the meltedportion 20 remains between the surfaces 1 a and 2 a to be joined.However, when they are held at a temperature not lower than the meltingpoint of the joining agent 3, portions 21 of the substrates 1 and 2 inthe area of the melted portion 20 convert to a liquid state. Referencenumeral 5 indicates a melted layer of the joining agent 3. When ajoining agent 3, for example, made of an oxide of a Ca—Al—O serieseutectic composition or a Y—Ca—Al—O series eutectic composition is used,it is considered that the melting of the joining agent begins at around1,415° C. or 1,375° C. and then the surfaces 1 a and 2 a of thesubstrates 1 and 2 contact with the melted joining agent to becomeliquefied.

Aluminum nitride-series ceramic materials usually experiences asintering process which is a so-called “liquid phase sintering”. Thatis, aluminum nitride-series ceramic materials experiences a process thatthe aluminum nitride particles are once liquefied and then solidifiedduring the cooling step. In the present invention also, aluminumnitride-series particles are considered to liquefy in the area of theinterfaces between the melted joining agent 3 and the substrates 1 and 2to form a liquid phase, and the components of the joining agent 3 arediffused through the liquid phase to change the composition of thatportion.

When the aluminum nitride particles are melted into the liquid phase,protruded portions of the surfaces 1 a and 2 a to be joined arepreferentially melted into the melted joining agent. Thus, the surfaces1 a and 2 a are flattened. If they are cooled thereafter withoutperforming the second process, the components of the joining agent areprecipitated at the crystal grain boundaries of the aluminum nitrideparticles.

In the melting process (first process), the heating is effected at atemperature of at least the melting point of the joining agent.Preferably the heating is effected below a temperature at which thelater described exhaust of the joining agent substantially occurs so asto avoid exhaust of the joining agent at this process.

Then, in the second process when the temperature is raised to atemperature higher than the first process, the melted joining agent issubstantially exhausted from between the substrates to provide acontinuous body of substrates without an intervening third phase. Thismechanism is similar to the mechanism of exhausting a sintering agent,such as yttrium etc., from liquefied aluminum nitride particles andexhausting further to the exterior of sinterd body of aluminum nitrideparticles from the interior of the sinterd body, at the time ofsolidifying aluminum nitride from a liquid phase to precipitate aluminumnitride particles.

The present invention is suitable for aluminum nitride-series ceramicmaterials having a relative density of at least 95%, and particularlysuitable for aluminum nitride-series ceramic materials having a relativedensity of at least 98%. The present invention is particularly suitablewhen at least one of the aluminum nitride-series ceramic materials is afired product produced by a hot press sintering process or a hotisotactic pressing process, because deformation or the like deficiencymay occur sometimes, when the present invention is applied to a calcinedbody or the like having a large specific surface area.

In the first and second processes, constant temperatures are preferablymaintained in the respective temperatures range of the processes.However, the temperature in the first and second processes may bechanged higher or lower in the respective temperature range of theprocesses. Particularly, the temperature range in the first process ispreferably at least 1,400° C. in order to securedly melt the joiningagent, and more preferably at least 1,450° C. in order to accelerate theliquefaction of the aluminum nitride particles at their interfacebetween the joining agent. If the exhaustion of the joining agentproceed in the first process, diffusion or invasion of the joining agentinto the substrates becomes difficult to proceed. Thus, in order toinhibit the exhaustion of the joining agent, the temperature in thefirst process is preferably not more than 1,650° C.

The temperature range in the second process is preferably at least1,650° C. in order to accelerate the exhaustion of the joining agent,preferably not more than 1,800° C. in order to prevent deformation andalteration etc. of the aluminum nitride-series substrates.

Heating time in the first and second processes is preferably at least 30min and not more than 10 hrs.

Atmosphere in the first and second processes can be an inert gas such asN2 etc. so far as it is a non-oxidizing atmosphere, or it may be vacuum.In the second process, nitrogen atmosphere is particularly preferable,because decomposition of aluminum nitride though in minor extent wasobserved in vacuum.

Preferably, a pressure is exerted as shown by the arrow A in FIG. 2b atthe time of joining, in order to further improve the joining strength.Practical effect of the exertion of pressure can be exhibited from apressure of at least 5 kg/cm² with an upper limit of 500 kg/cm².Exertion of a pressure exceeding the upper limit tends to easydeformation or cracks in the substrates. If the pressure is exerted at alow temperature, the substrate is liable to split. Thus, the pressure ispreferably exerted at a temperature of not lower than the melting pointof the joining agent.

Next, the joining agents which can particularly satisfactorily be usedin the present invention will be explained. The inventors have found outthat the joining agents are not specifically limited, however, thosejoining agents having an X—Y—Z series composition are particularlypreferable, wherein X is a compound of at least one metallic elementselected from the group consisting of alkali metal elements and alkalineearth metal elements, Y is a compound of rare earth elements, and Z is acompound of aluminum. Among the all metallic elements constituting thejoining agent, the proportion of the metallic elements constituting X is25-50 mol %, the proportion of the rare earth elements constituting Y is5-30 mol %, and the rest is aluminum.

In the aluminum nitride-series substrates, exhaustion of the joiningagent proceeds at a temperature exceeding 1,6500° C. and the joiningagent can hardly invade into the aluminum nitride at such hightemperatures. Therefore, the joining agent is selected so as to have amelting point of not higher than 1,650° C. and preferably a meltingpoint of not higher than 1,600° C. In the present invention, theexpression “melting point of the joining agent” means a temperature atwhich the liquid phase begins to form.

Rare earth elements used herein means seventeen elements of scandium,yttrium, lanthanum, cerium, praseodymium, neodymium, promethium,samarium, europium, gadolinium, terbium, dysprosium, holminm, erbium,thulium, ytterbium and lutetium. Among these elements, yttrium,lanthanium, cerium, neodium and ytterbium have particularly high effectof exhausting the joining agent, and yttrium and ytterbium are morepreferable, and yttrium is most preferable.

As the metallic elements constituting X, lithium, calcium, strontium andbarium are particularly preferable.

As the compounds of X, the compounds of Y and the compounds of Z,embodically oxides or fluorides are preferable. Compounds other thanoxides and fluorides can be used, however, compounds which can produceoxide or fluoride at the time of melting the joining agent arepreferable in such circumstances. As such compounds, carbonates,nitrates, oxalates and phosphonates are mentioned.

Preferable compositions of the joining agent are exemplified above, andthe joining agent includes the followings:

(1) Mixtures composed of compound of X, compound of Y and compound of Z.In these cases, the above-described oxides, fluorides, carbonates,nitrates, oxalates and phosphonates etc, can be uses as the compound ofX, compound of Y and compound of Z.

(2) Compounds containing all the components X, Y and Z. For example, anoxide of a metal constituting X, an oxide of a metal constituting Y andan oxide of a metal constituting Z can be mixed to obtain a mixture, andthe mixture can be calcined or fired to obtain a complexed compound or aglass. The complexed compound or the glass can be used as the joiningagent.

The joining agent may have a shape of a mixed powder, a calcined powder,a foil or a flat plate.

Among the joining agents, those joining agents wherein at least one ofthe component X and the component Y contains an oxide or a fluoridehaving a vapor pressure of 0.001-1,000 Pa at 1,650-1,800° C. arepreferable. As concrete examples of such oxide or fluoride, Li₂O, MgO,CaO, SrO, BaO and SrF₂ may be mentioned.

Particularly preferable are joining agents of a series of 25-40 wt % ofCaO, 15-30 wt % of Y₂O₃ and the rest of Al₂O₃. Among these joiningagents, joining agents of an eutectic composition of 37 CaO—19 Y₂O₃—44Al₂O₃ (melting point is 1,375° C.) and an eutectic composition of 28CaO—26 Y2O₃—46 Al₂O₃ (melting point is 1, 395° C.) are particularlypreferable.

FIG. 3 is a schematic cross-sectional view of a structure for holding asuspect 6 for use in the production of semiconductors, and FIG. 4 is aschematic partial cross-sectional view of the structure of FIG. 3 alongthe line IV—IV. The susceptor 6 has a shape of disc, for example, and asemiconductor wafer can be provided on the front surface 6 b of thesusceptor 6. To the back surface 6 a of the susceptor 6 is joined an endsurface 8 a of a holding member 8 of, for example, a substantiallycylindrical shape. The susceptor 6 and the holding member 8 are bothmade of aluminum nitride series material and joined to each otheraccording to the present invention. Referential numeral 7 is athroughhole for inserting a lift pin. Preferably, when forming a flangeportion 8 b at around the end surface 8 a of the holding member 8, apressure can be exerted on a peripheral edge surface 8 c of the flangeportion 8 b, as shown by the arrow B in FIG. 3.

Function and structure of the susceptor 6 are not limited specifically,and illustrative examples thereof are ceramic electrostatic heaterhaving a heat-generating resistive body embedded in the aluminum nitrideseries-substrate, a ceramic electrostatic chuck having an electrode forthe electrostatic chuck embedded in the substrate, a heater with anelectrostatic chuck having a heat-generating resistive body and anelectrode for the electrostatic chuck embedded in the substrate and anelectrode apparatus for generating a high frequency wave having aplasma-generating electrode embedded in the substrate.

For instance, in the apparatus shown in FIGS. 3 and 4, a heat-generatingresistive body not shown is embedded in the susceptor 6 and cables 10are connected to terminals 12 of the heat-generating resistive body. Inthe susceptor 6 is embedded a not shown plate-shaped electrode whichfunctions as a plasma-generating electrode or an electrode ofelectrostatic chuck, and a cable 11 is connected to the terminal 13 ofthe electrode. These cables 10, 11 and the terminals 12, 13 are allaccommodated in the inner space 9 of the holding member 8 and do notdirectly contact to a corrosive gas or plasma thereof in the chamber ofthe semiconductor production apparatus.

In addition, the present invention can be used for joining a susceptorfor mounting a semiconductor wafer, a damy wafer, a shadow ring, a tubefor generating a high frequency plasma, a dome for generating a highfrequency plasma, a high frequency wave-permeating window, an infraredwave-permeating window, a lift pin for supporting a semiconductor waferor a shower plate, etc. to another member.

DESCRIPTION OF PREFERRED EMBODIMENTS

Hereinafter, the present invention will be explained in more detail withreference to concrete experimental results.

COMPARATIVE TEST 1

Aluminum nitride blocks 1, 2 having a size of 20 mm×20 mm×10 mm as shownin FIG. 1a were ground at joining surfaces 1 a, 2 a by a No. 800grinding stone. At that time, one block had a purity of 95% (5% wasyttria), while the other block had a purity of 99.9%.

Between the two blocks was interposed a powder mixture prepared to havea composition in weight basis of 37 CaO—19 Y₂O₃—44 Al₂O₃ (having aneutectic point of 1,375° C.). The prepared sample was put in an electricfurnace, heated up to 1,500° C., held for 2 hrs and left to cool in thefurnace. The heating was carried out in a nitrogen atmosphere. Duringthe heating, a pressure of 50 kgf/cm₂ was continuously exerted on thejoining surfaces. A cross-section of the sample was observed by aphotograph taken by a survey type microscope and a reflex electronimage. FIG. 6 is a photograph of a reflex electron image showing aceramic tissue adjacent the joining interface of the joining body.

In the photograph shown in FIG. 6, the ceramic tissue is seen toseparate into three layers of an aluminum nitride layer of a purity of95%, a joining agent layer and an aluminum nitride layer of a purity of99.9%, as shown from the left side. In the left aluminum nitride layer,white crystal grain boundary layers consisting mainly of yttria are seenat black crystal grain boundary layers of aluminum nitride particles. Inthe joining agent layer, white needle-shaped crystals and a gray matrixsurrounding the white crystals can be observed. Though both the whiteneedle-shaped crystals and the gray matrix were produced from thejoining agent having an eutectic composition of 37 CaO—19 Y₂O₃—44 Al₂O₃,they are considered to have different compositions from each other. Theright aluminum nitride layer consists almost of black aluminum nitrideparticles and a few crystal grain boundary layers.

In the neighborhood of the interface between the left aluminum nitridelayer and the joining agent layer, the crystal grain boundary layers ofaluminum nitride particles are not white (showing the presence ofyttria) but pose the same color with the matrix of the joining agentlayer, (thus showing the invasion of the joining agent into the crystalgrain boundary layers.) Thickness of the invaded portion of the joiningagent was reached about 10 μm. This is because, at the time of theprocess of heating and holding at 1,500° C., aluminum nitride particleswere melted at the neighborhood of the interface between the substrateand the melted joining agent and the components of the joining agentwere diffused.

Also, in the neighborhood of the interface between the right aluminumnitride layer and the joining agent layer, invasion of the joining agentof 0.5 μm or more was seen in the crystal grain boundary layer of theAlN particles.

COMPARATIVE TEST 2

The processes of Comparative Test 1 were repeated except that thecompositions of the joining agent were changes to CaO, CaF, Y₂O₃,Y(NO₃)3 or YN to perform the above experiments, respectively. As aresult, substrates 1 and 2 could not be joined in all the examples.

Test 3

Experiment Nos. 1-7 included in the scope of the present invention asshown in Table 1 were put into practice.

Aluminum nitride blocks 1, 2 having a size of 20 mm×20 mm×10 was shownin FIG. 1a were ground at joining surfaces 1 a, 2 a by a No. 800grinding stone. At that time, one block had a purity of 95% (5% wasyttria), while the other block had a purity of 99.9%.

Powders of CaO, Y₂O₃ and Al₂O₃ of specific reagent grades were mixed inethanol in a weight % ratio of 37 CaO—19 Y2O₃—44 Al₂O₃ to obtain a mixedpowder of an eutectic point of 1, 375° C. Then, the mixed powder washeated in air at 1,000° C. for 2 hrs to obtain a calcined powder. Thecalcined powder was adjusted to have a maximum particle diameter of 100μm. The adjusted powder was interposed between the surfaces of the twosubstrates to be joined. Applied amount of the joining agent was 1-20mg/cm².

The thus obtained sample was put in an electric furnace and heated basedon a temperature—pressure schedule as shown in FIG. 5, wherein t0-t7represent elapsed heating time, respectively, T1-T4 represent heatingtemperature, respectively, and P1 and P2 are pressures. When thetemperature reached to T1=1,400° C. at the time of t1, the pressure wasbegan to exert, and when the temperature reached to T1=1,430° C. at thetime of t2, the pressure was increased to the respective value as shownin Table 1. By heating for a time of t3, the sample reached to atemperature T3, wherein T3 is a holding temperature of the sample at thefirst holding process, and a difference between the times t4 and t3 is aholding time at the temperature T3.

Then, the temperature of the sample was raised to a temperature T4 fromthe time t4 to the time t5, wherein T4 is a holding temperature of thesample at the second holding process, and a difference between times t6and t5 is a holding time at the temperature T4. Joinings of thesubstrates were put into practice based on such a temperature—pressureschedule, and the thus obtained joined bodies were measured onrespective properties. The measured results are shown in Table 1.

“Joining strengths” were measured by a method of measuring a four-pointbending strength according to Japanese Industrial Standard (JIS) R 1601,however, the specimen prisms for the bending tests were processed tohave the joining interface at the center thereof. The joining strengthswere measured at room temperature. Cross-sections of the specimens wereobserved by a survey type electron microscope to find residual levels ofthe components of the joining agent. With regard to “the residuum of thethird layer”, the joining interfaces were observed and thicknesses ofthe third layer were measured. Meanwhile, “thickness of the appliedjoining agent layer” were calculated from a formula of appliedamount/theoretical density of the joining agent/joining area, and valuesobtained by dividing “thickness of the third layer” by “thicknesses ofthe applied joining agent layer” were shown in Table 1.

The values of “leakage amount” in Table 1 were measured as follows. Around disc of a diameter of 50 mm and a thickness of 15 mm was used asone substrate, while a round pipe of an outer diameter of 36 mm, aninner diameter of 28 mm and a length of 10 mm was used as the othersubstrate. The round disc and the round pipe were joined as shown inFIG. 4, using the same joining method as described above. The thusobtained joined body was subjected to a helium leakage test. Measuringlimit of the used testing machine was 10×1.0⁻⁸ torr ·1/sec.

TABLE 1 1 2 3 4 5 6 7 Purity Substrate 1 99.9% 95% 95% 95% 95% 95% 95%of AlN Substrate 2 99.9% 99.9% 95% 99.9% 99.9% 99.9% 99.9% JoiningComposition 37CaO—19Y₂O₃—44Al₂O₃ agent Shape Calcined powder Temperatureand time in 1,550° C. 1,550° C. 1,600° C. 1,550° C. 1,650° C. 1,550° C.1,550° C. the first holding process 2 hrs 2 hrs 2 hrs 2 hrs 2 hrs 2 hrs2 hrs Temperature and time in 1,700° C. 1,700° C. 1,700° C. 1,700° C.1,750° C. 1,700° C. 1,700° C. the second holding process 2 hrs 2 hrs 2hrs 2 hrs 2 hrs 2 hrs 2 hrs Atmosphere N₂: 1.5 N₂: 1.2 N₂: 1.2 vacuumN₂: 1.2 N₂: 1.5 N₂: 1.8 atm atm atm atm atm atm Pressure (kgf/cm²) 50 5050 25 350 22 0 Joining strength (MPa) 440 220 160 100 250 130 100 Amountof leakage <10⁻⁸ <10⁻⁸ <10⁻⁸ <10⁻⁷ <10⁻⁸ — <10⁻⁷ Residuum of third layernone none none none none none none

As seen from the above Table 1, all the samples obtained in theExperiments 1-7 of the embodiments of the present invention couldprovide extremely high joining strengths, while no residuum of the thirdlayer was seen and the leakage amount was small. Among these, thejoining body of Experiment 1 using aluminum nitride sintered bodies of apurity of 99.9% as the substrates 1, 2 could exhibit the most highstrength. In Experiments 2-7, as the pressure became higher, the joiningstrength became increased. Also, it was found out that the joiningsperformed in nitrogen atmosphere can provide higher joining strengthsand extremely smaller leakage amount than the joining performed invacuum in Experiment 4.

After the measurements of the leakage amount, a heating cycle test of100 cycles of heating between 50° C. and 700° C. in air was put intopractice to judge whether the joined body can withstand such a thermalshock or not. In Experiments 1-7, no defect was found and nodeterioration of the leakage amount was found. In Table 1, the specimensof Experiments 1-3 showed no deterioration of leakage amount though theywere exposed in NF₃ plasma at 450° C. for 24 hrs.

Table 7 is a reflex electron image showing a ceramic tissue at theneighborhood of the joining interface of the specimen obtained byExperiment 2 shown in Table 1. The ceramic tissue is seen from the upperside to separate into two layers of an aluminum nitride layer of apurity of 95% and an aluminum nitride layer of a purity of 99.9%. In theupper aluminum nitride layer, white crystal grain boundary layersconsisting mainly of yttria are seen at the crystal grain boundary ofblack aluminum nitride particles. In the lower aluminum nitride layer,yttria is not observed.

In the neighborhood of the interface between the upper and loweraluminum nitride layers, the sizes of aluminum nitride particle and thesizes of crystal grain boundaries in the upper and lower layers differcertainly, so that a distinct boundary can be observed. In the boundary,no second phase or crack is existent to understand that the upper andlower aluminum nitride tissues are directly continuously joined.

FIG. 8 is a secondary electron image showing the ceramic tissue at theneighborhood of the joining interface of the bending test specimen afterthe above described heating cycle test. The secondary electron image isseen from the upper side to separate into two layers of an upperaluminum nitride layer of a purity of 95% and a lower aluminum nitridelayer of a purity of 99.9%. At the interface between the two layers, nocrack or deteriorated layer can be seen.

FIG. 9 is a secondary electron image showing the ceramic tissue at theneighborhood of the joining interface of the bending test specimenobtained by Experiment 1 shown in Table 1. In the neighborhood of theinterface between the upper and lower aluminum nitride layers, adistinct boundary cannot be observed and no secondary layer or crack isexistent at all to understand that the upper and lower aluminum nitridetissues are directly and continuously joined.

Test 4

In the same manner as described in Test 3, joined bodies (comparativeexamples) of respective Experiment Nos. 8-12 as shown in Table 2 wereproduced. Conditions of respective experiments and the results are shownin Table 2.

TABLE 2 8 9 10 11 12 Purity of AlN Substrate 1 95% 95% 95% 95% 95%Substrate 2 99.9% 99.9% 99.9% 99.9% 99.9% Joining agent Composition37CaO—19Y₂O₃—44Al₂ Shape Calcined powder Temperature 1,550° C. none1,400° C. 1,700° C. 1,300° C. and time in 2 hrs 2 hrs 2 hrs 2 hrs thefirst holding process Temperature none 1,700° C. 1,500° C. 1,900° C.1,700° C. and time in 2 hrs 2 hrs 2 hrs 2 hrs the second holdingAtmosphere N₂:1.5 N₂:1.5 N₂:1.5 N₂:1.5 N₂:1.5 atm atm atm atm atmPressure 50 50 50 50 50 (kgf/cm²) Joining 130 20 80 60 30 strength (MPa)Amount of <10⁻⁶ <10⁻⁶ <10⁻⁵ <10⁻⁵ <10⁻⁵ leakage Residuum of 70% none 80%none none third layer

As seen from Table 2, a comparative sample was held at 1,550° C. for 2hrs to obtain a somewhat high joining strength in Experiment 8, however,it showed a leakage amount of a level of 10⁻⁶ and residuum of a thirdlayer. When the sample was subjected to a heating cycle test, thejoining interface was peeled away. In Experiment 9, a comparative samplewas held at 1,700° C. for 2 hrs, however, the joining strength was astill insufficient value of 20 MPa and a leakage amount was a level of10⁻⁶. The joining agent was exhausted during the heating process and theresiduum of a third layer was not seen. In Experiment 10, a comparativesample was held at 1,400° C. for 2 hrs and held at 1,500° C. for 2 hrs,however, the exhaust of a third layer was not seen substantially and thejoining agent remained 80% and the leakage amount was a level of 10⁻⁵.

In Experiment 11, a comparative sample was held at 1,700° C. and 1,900°C., respectively for 2 hrs and showed no residuum of a third layer.However, the joining strength was still insufficient and the leakageamount was a level of 10⁻⁵. After subjected to the heating cycle test,the substrates were found to have deformed. In Experiment 12, acomparative sample was held at 1,300° C. and 1,700° C., respectively for2 hrs and showed no residuum of a third layer. However, the joiningstrength was still insufficient and the leakage amount was a level of10⁻⁵. In Experiments 11, 12, the invasion process of the melt into thesubstrates was considered not to progress.

Test 5

In the same manner as described in Test 3, joined bodies (examples ofthe present invention) of the respective Experiment Nos. 13-17 as shownin Table 3 were produced. Conditions of respective Experiments andmeasured results are shown in Table 3. However, a mixed powder beforecalcining was used in Experiments 13, 14 and 15, and a ground powderobtained by heat treating the mixed powder at 1,360° C. for 2 hrs andthen grinding in a vibration mill was used in Experiment 16. InExperiment 17, a foil-shaped sintered body was used obtained by heattreating the mixed powder at 1,360° C. for 2 hrs and then processing atthat state into a foil shape of a thickness of 0.5 mm.

TABLE 3 13 14 15 16 17 Purity of AlN Substrate 1 99.9% 95% 95% 95% 95%Substrate 2 99.9% 99.9% 95% 99.9% 99.9% Joining agent Composition37CaO—19Y₂O₃—44Al₂O₃ Shape Mixed powder Ground Sinter powder bodyTemperature 1,550° C. 1,400° C. 1,650° C. 1,550° C. 1,550° C. and timein 4 hrs 4 hrs 1 hr 8 hrs 2 hrs the first holding process Temperature1,700° C. 1,700° C. 1,800° C. 1,660° C. 1,700° C. and time in 4 hrs 2hrs 30 min 8 hrs 2 hrs the second holding process Atmposphere N₂:1.5N₂:1.2 N₂:1.2 N₂:3.0 N₂:1.2 atm atm atm atm atm Pressure 50 50 50 480 50(kgf/cm²) Joining 440 120 200 350 250 strength (MPa) Amount of <10⁻⁸<10⁻⁸ <10⁻⁸ <10⁻⁸ leakage Residuum of none none none none none thirdlayer

As seen from these examples, in either case of using a calcined powder,a powder obtained by grinding the calcined powder or a foil-shapedsintered body, substantially the same results as the case of using thecalcined powder could be obtained.

Test 6

In the same manner as described in Test 3, joined bodies of respectiveExperiment Nos. 18-21 as shown in Table 4 were produced. Conditions ofrespective Experiments and measured results are shown in Table 4.However, in Experiment 18 (example of the present invention) a groundpowder of a composition of 28 CaO—26 Y₂O₃—46 Al₂O₃ was used, and inExperiments 19, 20 and 21 (comparative examples) a powder of Y₂O₃, YN or80 YF—20 AlF₂ was used.

TABLE 4 18 19 20 21 Purity of AlN Substrate 1 95% 95% 95% 95% Substrate2 95% 99.9% 95% 99.9% Joining agent Composition 28CaO- Y₂O₃ YN 80YF₃-26Y₂O₃- 20AlF₃ 46Al₂O₃ Shape Ground Powder Powder Mixed powder powderTemperature and time 1,550° C. 1,550° C. 1,550° C. 1,550° C. in thefirst holding 3 hrs 2 hrs 2 hrs 2 hrs process Temperature and time1,800° C. 1,700° C. 1,600° C. 1,700° C. in the second holding 1 hr 2 hrs2 hrs 2 hrs process Atmosphere N₂:1.2 N₂:1.2 N₂:1.2 N₂:1.2 atm atm atmatm Pressure (kgf/cm²) 50 50 50 50 Joining strength 160 0 0 0 (MPa)Amount of leakage <10⁻⁸ — — — Residuum of third none 100% — — layer

In Experiment 18 a joined body of a high joining strength and aleakage-proving property was obtained. However, in Experiment 19 Y₂O₃was not at all exhausted and a joined body could not be obtained. InExperiments 20 and 21 the respective powder was not melted to form ajoining layer and a joined body could not be obtained.

Test 7

Joined bodies as shown in FIGS. 3 and 4 were produced. However, in thesecases, in the susceptor 6 were embedded a molybdenum coil whichfunctions as a heater and a molybdenum mesh which functions as anelectrode of a high frequency wave plasma. The sintered body 6 had apurity of 99.9% and relative density of at least 99.5% and was producedby a hot press method.

Also, an annular holding member 8 made of a normal pressure sinteredbody of an outer diameter of 60 mm, an inner diameter of 52 mm and alength of 210 mm (purity 95% andyttria 5%) was prepared. The susceptor 6and the holding member 8 were joined according to the present invention.

In the furnace for performing the joining, a hot press made of a carbonmaterial for furnace was used. Heating was effected in a nitrogenatmosphere of a pressure of 1.5 atm, and a pressure of 60 kgf/cm² wasexerted on the joining surface by a hydraulic oil press, while thesample was heated to a temperature of at least 1,300° C. The sample washeated at a temperature elevating rate of 1,00-1000° C./hr, held at1,550° C. for 2 hrs to cause the joining agent to invade into thejoining surfaces of the substrates, and then heated at 1,700° C. for 2hrs to exhaust the joining agent. After heated at 1,700° C. for 2 hrs,the sample was left cool in the furnace.

The joined body was taken out from the furnace and the amount of leakagewas measured in the same manner as described above. As a result, theamount of leakage was less than 1.0×10⁻⁸ torr ·1/sec.

To the molybdenum coil and molybdenum mesh embedded in the interior ofthe sintered body were attached lead wires of electrodes. The coil andmesh were heated by passing an electric current therethrough to repeat30 cycles of heat elevation and heat lowering at a rate of about 25°C./min. As a result, no crack or deformation of the joined body wasfound. After the heating cycle test, the sample was tested again by ahelium leakage test to find again a leakage amount of less than 1.0×10⁻⁸torr ·1/sec.

Test 8

In the same manner described in Test 3, joined bodies (examples of thepresent invention) of respective Experiment Nos. 22-25 as shown in Table5 were produced. Conditions of respective Experiments and measuredresults are shown in Table 5.

TABLE 5 22 23 24 25 Purity of AlN Substrate 1 99.9% 95% 95% 95%Substrate 2 99.9% 99.9% 95% 99.9% Joining agent X Li₂O Li₂O Li₂O BaO 49mol % 49 mol % 49 mol % 56 mol % Y Y₂O₃ Y₂O₃ Y₂0₃ Y₂O₃ 26 mol % 26 mol %26 mol % 7 mol % Z Al₂O₃ Al₂O₃ Al₂O₃ Al₂O₃ 25 mol % 25 mol % 25 mol % 37mol % Shape Calcined Calcined Calcined Calcined powder power power powerTemperature and time in 1,500° C. 1,550° C. 1,550° C. 1,500° C. thefirst holding process 2 hrs 2 hrs 2 hrs 2 hrs Temperature and time in1,620° C. 1,650° C. 1,650° C. 1,650° C. the second holding 1 hr 2 hrs 2hrs 2 hrs process Atmosphere N₂:2.5 N₂:1.7 N₂:1.7 N₂:2.5 atm atm atm atmPressure (kgf/cm²⁾ 20 20 20 20 Joining strength (MPa) 300 180 120 250Amount of leakage <10⁻⁸ <10⁻⁸ <10⁻⁸ <10⁻⁸ Residuum of third layer nonenone none none

In Experiments 22, 23 and 24, a Li₂O—Y₂O₃—Al₂O₃ series calcined powderwas used as the joining agent. All of them showed high joining strength,small leakage amount and no residuum of a third phase. From theviewpoint of joining strength, preferably an aluminum nitride substrateof a relative density of 99.9% is used and the pressure of N₂ atmosphereis increased to 2.5 atm, as in Experiment 22.

In Experiment 25, a BaO—Y₂O₃—Al₂O₃ series calcined powder was used asthe joining agent. The sample showed high joining strength and smallleakage amount.

The samples of Experiments 22-25 were after the measurement of theleakage amount subjected to a heating cycle test of 100 cycles ofheating the samples between 50° C. and 700° C. in air. As a result, allthe samples showed no defect and no deterioration of the leakage amount.The bending test specimen of Experiment 22 was exposed in N₂ plasma at450° C. for 24 hrs to find no deterioration the joining of strength andthe leakage amount.

Test 9

In the same manner as described in Test 3, joined bodies (examples ofthe present invention) of respective Experiment Nos. 26-30 as shown inTable 6 were produced. Conditions of respective Experiments and measuredresults are shown in Table 6.

TABLE 6 26 27 28 29 30 Purity of AlN Substrate 95% 95% 95% 95% 95% 1Substrate 99.9% 99.9% 99.9% 99.9% 99.9% 2 X CaO CaF₂ CaO CaO BaO 56 mol% 56 mol % 55 mol % 48 mol % 45 mol % Y La₂O₃ Y₂O₃ YF₃ Y₂O₃ La₂O₃ 7 mol% 7 mol % 14 mol % 9 mol % 5 mol % Z Al₂O₃ 37 mol % 37 mol % 31 mol % 0mol % 25 mol % AlF₃ 0 mol % 0 mol % 0 mol % 43 mo % 25 mol % ShapeCalcined Calcined Calcined Calcined Calcined powder powder powder powderpowder Temperature 1,550° C. 1,550° C. 1,550° C. 1,550° C. and time in 2hrs 2 hrs 1 hr 1 hr 1 hr the first holding process Temperature 1,620° C.1,650° C. 1,700° C. 1,700° C. 1,700° C. and time in 1 hr 2 hrs 2 hrs 2hrs 2 hrs the second holding process Atmosphere N₂:1.7 N₂:1.7 N₂:1.5N₂:1.5 N₂:1.5 atm atm atm atm atm Pressure 20 20 30 30 30 (kgf/cm²)Joining 190 140 100 130 180 strength (MPa) Amount of <10⁻⁸ <10⁻⁸ <10⁻⁸<10⁻⁸ <10⁻⁸ leakage Residuum of none none none none none third layer

As the joining agent, a CaO—La₂O₃—Al₂O₃ series calcined powder was usedin Experiment 26, a CaF₂—Y₂O₃—Al₂O₃ series calcined powder was used inExperiment 27, a CaO—YF₃—Al₂O₃ series calcined powder was used inExperiment 28, a CaO—Y₂O₃—Al₂O₃ series calcined powder was used inExperiment 29, and a BaO—La₂O₃—Al₂O₃ series calcined powder was used inExperiment 30. All the samples obtained in these Experiments showed highjoining strength, small leakage amount and no existence of a thirdphase.

As explained in detail in the foregoings, according to the presentinvention, aluminum nitride series substrates can be joined mutually inthe substantial absence of an intervening third layer at the joininginterface of the substrates, when joining the aluminum nitride seriessubstrates to each other.

Although the present invention has been explained with specific examplesand numeral values, it is of course apparent to those skilled in the artthat various changes and modifications thereof are possible withoutdeparting from the broad spirit and aspect of the present invention asdefined in the appended claims.

What is claimed is:
 1. A method for producing a joined body of aluminumnitride-series substrates, comprising a first step of interposing ajoining agent between the substrates to be joined, and heating thesubstrates and the joining agent at a temperature range of not less thanthe melting point of the joining agent to thereby melt the joining agentand liquefying particles of the aluminum nitride-series substrates to aliquid phase adjacent the interface between the melted joining agent andthe substrates, and a second step of heating the substrates and thejoining agent at a temperature range of higher than the temperaturerange of the first step and lower than the sintering temperature of thesubstrates to thereby exhaust the joining agent from between thesubstrates.
 2. The method for producing a joined body of aluminumnitride-series substrates as defined in claim 1, wherein a pressure isexerted on the joining agent and the substrates in the first and secondsteps.
 3. The method for producing a joined body of aluminumnitride-series substrates as defined in claim 1, wherein the joiningagent comprises an X-Y-Z composition, wherein X is a compound of a tleast one metallic element selected from the group consisting of alkalimetal elements and alkaline earth metal elements, Y is a compound ofrare earth element, and Z is an aluminum compound, and among themetallic elements constituting the joining agent the proportion of themetallic element constituting X is 25-50 mol %, the proportion of therare earth element constituting Y is 5-30 mol %, and the rest comprisesaluminum; and wherein during melting of the joining agent in the firststep, the melted joining agent comprises at least one of oxide andfluoride of the metallic elements constituting the compound X, at leastone of oxide and fluoride of the rare earth elements and at